Many industries require determination of a dimension of a package which is being transported from one location to another. Preferably, dimensional measurements are best made without interrupting or diminishing the movement of the package to be measured. It is therefore desired to measure the dimension of the packages at the rate they may move, and on the normal path of transit, such as on a conveyor.
A package height detector typically uses a vertically disposed column of closely spaced photocells on one side of the package and a parallel (vertical) light source on the other. The package to be measured interrupts the light (from the light source) impinging on all the photocells in the array that occupy a position corresponding to the bottom and the top of the package.
The bottom of the package is located spatially vertically at a reference plane, for example the horizontally moving surface of a conveyor belt. The column of photocells is located spatially such that its lowermost photocell is aligned approximately at or slightly below the said reference plane.
To simplify circuitry, the photocells are connected in a matrix such that a group of photocells can be interrogated selectively and the light intensity impinging on each cell of the group measured simultaneously. The light intensity is further quantified digitally, such that light intensity falling on a photocell above a certain threshold represents a binary one and light below said threshold represents a binary zero. The threshold level is chosen to be such that it is below the light intensity impinging on the photocells with no package intervening and above the light intensity impinging on the photocells with a package intervening. In this way, a binary one represents the absence of a package intervening between the light source and the photocell and a binary zero represents the presence of such an element. The one and zero definitions are arbitrary, and could readily be oppositely defined.
The sum of all the photocells operating in binary state zero with no package intervening then represents the height of the conveyor reference plane above the bottom of the photocell column. The sum of all the photocells in state zero with a package intervening then represents the height of the reference plane plus the height of the package. The physical height of the package is then determined by subtracting the total sum from the reference sum and multiplying this difference by the physical spacing between photocells, when the spacing between photocells is maintained constant.
In general, the purpose of making an optical measurement of height is to provide dynamic information to a process. The actual package height information is determined from the photocells through processing the photocell information. Quantifying the information into binary form is the first step in the processing.
The next step in processing depends on the grouping of the photocells in groups within the column of photocells. The groups can be arranged in any order, e.g. vertically ascendant. Photocells can grouped in groups of any number and the groups arranged in any order. If, for example, a 16 bit word computer is in use and the photocells are grouped in clusters of 16 photocells per group, then in vertically ascendant order, the first group would comprise photocells numbered 0 to 15, the second group photocells numbered 16-31, etc. Thus, to determine the height of a package, different groups are interrogated in a sequential order, such as vertically ascendant group interrogation, and the information from each cell in the group is analyzed. Therefore, if all the cells of a group contain binary zeroes, it means that the top of the package exists at a level above that of the group from which the data has just obtained, and the next group above will be interrogated in order to find out if the top exists at that level. Processing continues group by group until a group is found wherein an upper element contains binary ones, from which the height of the package can then be calculated. The number of groups that must be interrogated with such a scheme varies greatly and is proportional to the height of the package.
To illustrate, assume a 160 element array equally spaced at 0.2 inches and arranged as 10 groups of 16. If a vertically ascendant order is chosen, then the absence of a package is always determined in one sample. A 3 inch high package will be measured in one sample, a 12 inch high package will take 4 samples and a 24 inch high package will take 8. Assume that a sample can be measured in 400 .mu.sec. The time to obtain a valid height will thus vary from 400 .mu.sec to 3.2 msec. If a conveyor is moving at 100 inches per second, this means that the package can move well over 1/4 inch just during the processing time to obtain a single data sample. If any processing is done on the sampled data point, for example digital smoothing, the package may have moved significantly before the processed data is even stable.
Alternate interrogation sequences may be used. For example, one might start from the top down. Under this interrogation sequence, no package at all will result in the most processing time and the highest (tallest) package will produce the least processing time. Another scheme might be to start in the middle of the array and proceed up or down based on the results obtained from the prior samples. In any of these schemes, the number of interrogations of the array to arrive at a valid result will always require a number of interrogations that varies with the height of the package. This means that the processing time is a variable, and hence that the sampling rate will vary if high resolution sampling is attempted. The resulting height measurement processing time, being variable, makes processing and integration with other process controls difficult, and if extended in time, will misread package heights or require slower conveyor rates.